In modern sound-reproduction systems can be achieved, through using several loudspeakers, that individual sound sources can be accurately localized in the space and that within the reproduction environment is produced the impression that one is within a simulated area, e.g. a stadium or a cathedral. Two different reproduction concepts can in principle be distinguished. In the conventional surround reproduction, also usual in the field of home entertainment, the localisation and spatial information is already mixed during the sound mixing operation into individual channels to be transferred discretely, a reproduction system comprised of several loudspeakers being used for reproducing the individual channels. The reproducing loudspeakers should be located at a predetermined position with respect to the reproduction environment, in order to achieve an optimum spatial impression.
More advanced systems, such as the space simulations based on wave-field synthesis, generate the control signals for the individual loudspeakers only during the reproduction, based on position information of a sound source with respect to the reproduction room and the spatial information of a reproduction environment to be simulated. Substantially more authentic results can thus be achieved as regards the localisation and the spatial impression, since the individual loudspeaker setup here can be taken into consideration during the reproduction, in order to generate in the reproduction environment a wave-front, which best represents the spatial impression to be simulated.
For a better understanding of the present invention, the wave-field synthesis technique will now be described more in detail.
A better natural spatial impression as well as a stronger enveloping during audio reproduction can be achieved by means of a new technology. The bases for this technology, the so-called wave-field synthesis (WFS; WFS=Wave-Field Synthesis), have been investigated at the Delft TU and presented for the first time in the late 80's (Berkhout, A. J.; de Vries, D.; Vogel, P.: Acoustic control by Wave-field Synthesis, JASA 93, 1993).
Due to the huge requirements of this method as regards computer power and transmission rates, the wave-field synthesis has, until now, only very seldom been used in practice. Only the progresses in the fields of the microprocessor technique and the audio coding now permit using this technology in concrete applications.
The basic idea of WFS is based on the application of the Huygens principle of the wave theory:
Each point detected by a wave is the starting point of an elementary wave, which spreads spherically and/or in a circular way.
Applied to the acoustics, by a large number of loudspeakers, which are arranged next to each other (a so-called loudspeaker array), can be reproduced any form of incoming wave-front. In the simplest case, of one punctual source to be reproduced and a linear arrangement of the loudspeakers, the audio signals of each loudspeaker must be supplied with a time delay and an amplitude modulation such that the radiated sound fields of the individual loudspeakers properly overlap. In the case of several sound sources, the contribution to each loudspeaker is calculated separately for each source and the resulting signals are added. If the sources to be reproduced are located in a virtual room with reflecting walls, the reflections must also be reproduced as additional sources by the loudspeaker array. Therefore, the complexity of the calculation strongly depends on the number of sound sources, the reflection properties of the room and the number of loudspeakers.
The advantage of this technique resides in particular in that a natural spatial sound impression is possible over a large area of the reproduction room. In contrast to the well-known techniques, the direction and distance of the sound sources are reproduced very accurately. To a limited extent, virtual sound sources can even be positioned between the real loudspeaker array and the listener.
The wave-field synthesis thus permits a correct reproduction of virtual sound sources over a large reproduction area. At the same time, it provides the sound mixer and sound engineer with new technical and creative potential also when creating complex sound landscapes. The wave-field synthesis (WFS or also sound-field synthesis), as it was developed in the 80's at the Delft TU, represents a holographic approach of the sound reproduction. The Kirchhoff-Helmholtz integral serves as a basis. This means that arbitrary sound fields can be generated within a closed volume by means of a distribution of monopole and dipole sound sources (loudspeaker arrays) on the surface of this volume. Details hereof can be found in M. M. Boone, E. N. G. Verheijen, P. F. v. Tol, “Spatial Sound-Field Reproduction by Wave-Field Synthesis”, Delft University of Technology Laboratory of Seismics and Acoustics, Journal of J. Audio Eng. Soc., vol. 43, no. 12, December 1995 and Diemer de Vries, “Sound Reinforcement by Wavefield Synthesis: Adaptation of the Synthesis Operator to the Loudspeaker Directivity Characteristics”, Delft University of Technology Laboratory of Seismics and Acoustics, Journal of J. Audio Eng. Soc., vol. 44, no. 12, December 1996.
In the wave-field synthesis, a synthesis signal for each loudspeaker of the loudspeaker array is calculated from an audio signal, which is sent by a virtual source at a virtual position, the synthesis signals being formed, as regards their amplitude and phase, so that a wave, which results from the overlapping of the individual sound waves output by the loudspeaker present in the loudspeaker array corresponds to the wave, which would proceed from the virtual source at the virtual position if this virtual source at the virtual position were a real source with a real position.
Several virtual sources are typically present at different virtual positions. The calculation of the synthesis signals is performed for each virtual source at each virtual position, so that a virtual source typically results into synthesis signals for several loudspeakers. Viewed from a loudspeaker, this loudspeaker thus receives several synthesis signals, which proceed from different virtual sources. An overlapping of these sources, which is possible due to the principle of linear superposition, then results into the reproduction signal actually sent by the loudspeaker.
The possibilities of the wave-field synthesis can be best exhausted as the loudspeaker arrays are closer, i.e. the more individual loudspeakers are arranged as close as possible to each other. In this way, however, the calculation performance necessitated from a wave-field synthesis unit also rises, since channel information must typically also be taken into consideration. This means in particular that an own channel is, in principle, present from each virtual source to each loudspeaker, and that it can happen, in principle, that each virtual source leads to a synthesis signal for each loudspeaker, or that each loudspeaker receives a number of synthesis signals, which is equal to the number of virtual sources.
Furthermore, it should be pointed out here that the quality of the audio reproduction increases with the number of loudspeakers made available. This means that the more loudspeakers are present in the loudspeaker array or arrays, the better and more realistic the audio reproduction quality becomes.
Spatial sound-reproduction systems such as the wave-field synthesis thus allow generating the sound within 360 degrees around the auditorium with optimal spatial resolution. Until now, these systems were used essentially for positioning discrete sound sources and for direct sound reproduction. To the signals of the sound sources thus generated can, in addition, be applied all well-known linear signal processing operations, such as e.g. adding reverberation. In spatial sound-reproduction systems such as wave-field synthesis (WFS), it is furthermore possible to generate spatial effects based on the direct sound. This occurs for example in space simulation, wherein the reproduction can, for efficiency reasons, be simplified to a limited number of spatial directions (plane waves).
In a very simple case of space simulation, identical parameters are used for describing the room for all spatial directions (diffuse reverberation) and space portions depending on the direction (early reflections) are generated automatically. Generating spatial effects is judicious not only when natural spatial effects are to be reproduced, since the basic possibilities of this kind of signal processing can also be used in other creative ways.
In wave-field synthesis, a room to be provided with sound is provided with sound by as many individual loudspeakers as possible, in order to permit the reconstruction of wave-fronts with optimum accuracy. For orientating the sound signals and generating a spatial impression, there are usually used a plurality of parameters, which are to be determined for each loudspeaker individually during downmixing of the sound signal.
As described above, the multi-channel sound-reproduction systems are characterised by an extraordinarily high complexity, so that the additional generation of spatial information or orientation information during downmixing of the sound necessitates generating a plurality of parameters, which describe for each loudspeaker individually the orientation information or additional linear signal processing steps (for generating sound effects). This description by means of a plurality of abstract mathematical parameter without any directly and intuitively detectable meaning is difficult to be controlled, in particular in wave-field synthesis systems.
For example, wave-field synthesis provides the possibility of freely positioning sound sources on a two-dimensional listening level. This occurs through synthesizing different wave-fronts depending on the position of the sound sources. User surfaces, such as they are presently used, use a point in a plane view of the two-dimensional listening level for positioning the sound source, the point representing the position of the sound source. Since in this approach the spatial position of the sound source is of course sufficiently visualised, but the sound-depth impression (spatial impression) can, in principle, however not be represented simultaneously in the visualisation, discrepancies occur between the real perception and the representation, so that only in a few exceptional cases a visual picture is available, which corresponds to the real sound impression or permits to conclude same.